Crab vs. Oyster: As Acidity Increases, Some Species May Win and Others Lose

Jeffrey Brainard

IN THE CHESAPEAKE BAY AND THE OPEN OCEAN, waters with rising acidity are poison for some species and tonic for others.

Those discoveries came from recent laboratory studies about how Bay species are affected by water with different levels of pH, the laboratory scale that describes acidity. (Lower pH readings correspond to higher acidity.) That's important because of predictions that the Bay and the open ocean will slowly become more acidic in coming decades. Water in parts of the Bay is already naturally more acidic than in the open ocean.

This water threatens to degrade the shells and skeletons of marine organisms. As pH in seawater falls, so does the level of a form of carbon (called carbonate) that the creatures need for building those structures. Their shells could grow smaller or even dissolve, making them more vulnerable to predators and threatening their survival.

The laboratory findings suggest, however, that decreases in pH may have quite different effects on different species — and not always bad ones. The disparity is highlighted by two of the Bay's most iconic and commercially important species, the Eastern oyster and the Atlantic blue crab. Water with higher acid does appear harmful to oysters' shells. The findings for blue crabs were different — and unexpected.

Justin Ries, a scientist at the University of North Carolina at Chapel Hill, studied the effects of acidified water on those two species as well as 16 other shell-builders, including clams. The researchers wanted to know how those organisms would be affected by carbon dioxide (CO2) that is building up in the atmosphere. The carbon dioxide in the sky can dissolve in the sea, which tends to lower its pH (see box, Chemistry 102: The pH Scale).

Ries and his team grew the marine creatures in a bank of aquarium tanks similar to those in pet stores but outfitted to bubble carbon dioxide into the water. The pH in some tanks corresponded to today's level of carbon dioxide. The pH in other tanks reflected levels of CO2 two and three times higher than amounts before the Industrial Revolution; those levels are projected to occur by the year 2100 if carbon dioxide in the atmosphere continues to increase at the rate seen in recent decades. Another set of tanks held a still bigger dose of CO2, representing 10 times the pre-industrial level, predicted to occur within the next millennium.

Acidity at the Bay's Bottom

Ries says it was relevant to study water with CO2 levels that high because they're not only part of a future scenario: they can be found today in sediments in the deepest reaches of the Chesapeake Bay, where blue crabs hibernate for the winter. Water at the Bay's bottom can have a lower pH than at the surface because the Bay is an estuary with a steady inflow of nutrients from streams and rivers. The nutrients feed algae that bloom in the summer and eventually sink to the Bay's bottom, where they decompose. The process of decomposition creates more carbon dioxide, a waste product of metabolism, and raises acidity.

Ries's interest in what is going on in the Chesapeake's sediments is more than purely academic. He grew up in Baltimore and spent summers at his grandfather's marina on Gunpowder Cove and at a summer home on Harris Creek in Maryland. In fact, he obtained the blue crabs he used in the study from the breeders at the Institute of Marine and Environmental Technology (part of the University of Maryland Center for Environmental Science) in Baltimore Harbor.

His laboratory experiments included a seemingly surprising result: the lower the pH in the tanks, the heavier and larger were the crabs' shells. Higher acidity seemed to help blue crabs grow bigger. "Honestly I did not expect it," he says.

Ries chalks this up to the blue crab's body type. Its outer shell or exoskeleton is covered by a substance, called chitin, that protects it from the corrosive effects of surrounding water. What's more, crabs appear able to regulate the pH of fluid inside that covering, keeping the level higher (or less acidic) than the surrounding water. That's important because organisms like crabs build their shells from calcium and a form of carbon called carbonate. The pH level affects the amount of carbonate available for the crab to incorporate into its shell: a higher pH leads to more carbonate, and a lower pH results in less. So when crabs control their internal pH, they can generate more raw material for shell building.

The blue crab probably evolved that capacity because it molts, Ries explains. Its survival depends on building and solidifying a new shell within days after it sheds its old one. So blue crabs needed a physiology that could maximize the amount of carbonate available to build their shells. That same mechanism can probably help blue crabs grow larger shells when carbon dioxide levels are higher than today's, Ries says. The effect is similar to what we see in modern humans, who grow taller on average than people who lived during the Middle Ages because today we enjoy diets richer in protein and calcium.

"We think that the crabs have evolved a sophisticated mechanism, not necessarily to prevent the effects of acidification, but just to go about their normal molting process," Ries says. "The carryover effect is that it makes them more resilient to acidification."

Different Oysters, Different Effects

Elevated levels of carbon dioxide and acidity affected the shells of marine animals in a study by Justin Ries of the University of North Carolina (above, with tanks used in his study). High levels­ of acidity hurt growth in Eastern oysters but helped it in Atlantic blue crabs. Surprisingly, crabs grown experimentally (above, right) at the highest levels of carbon dioxide (10 times pre-industrial levels, right) developed larger shells than those grown at today's levels (above, left). The scale is in centimeters after 60 days of growth. Photographs courtesy of Justin Ries.

Other Bay species tested by Ries and his colleagues included hard- and soft-shell clams, and he found that their shells actually dissolved at the highest levels of carbon dioxide. That's an effect of acidification that doesn't offer good news for the blue crab, because clams are among its prey. It's an example of why scientists say that the effects of acidification have to be studied holistically within ecosystems, not just species by species.

When Ries tested Eastern oysters (Crassostrea virginica) from Cape Cod, he found effects that were less severe than in clams but still significant: the rate of growth in its shell steadily declined as the water's pH fell. Other researchers studying the Eastern oyster have found similar effects (see separate article, Shell Game).

At least one species of oyster, it turns out, can survive in higher acidity waters. When Whitman Miller ran laboratory experiments at the Smithsonian Environmental Research Center, he found that the Asian oyster (Crassostrea ariakensis) showed no loss of shell when exposed to higher acidity levels. Scientists think that the Asian oyster, native to the rivers of China, is better adapted to low pH waters because it evolved under more acidic conditions.

Oysters' bodies and shells are different from crabs' in several important respects. Oysters have a protective covering (called the periostracum), but it doesn't completely cover their shells. And unlike crabs, they don't molt. An oyster builds its shell continuously.

Other researchers have looked beyond shellfish to study other species that dwell in coastal waters. A study of moon jellyfish by researchers at Western Washington University showed they reproduced just fine in highly acidified water. And although underwater grasses might be expected to fare well in water rich with carbon dioxide, which they photosynthesize, they instead appear to sustain damage under some circumstancess (see separate article, An Acidifying Estuary?).

A big caveat to these laboratory studies, acknowledged by Ries and other researchers, is that they may not accurately predict what would happen to those same creatures in the natural environment of the Bay. Ries gave the oysters a constant dose of low pH for two months, something they would not experience in nature, where pH levels fluctuate. In the Bay, the creatures may be able to compensate and build their shells despite these unfavorable conditions — although at a cost in energy that could reduce their survival, scientists say.

Still, these studies are significant because some earlier reports about acidifying water have implied that lower pH puts all shell-building organisms at risk. Ries and other scientists have offered a more nuanced picture, one that highlights the importance of differences in adaptations among different species. The impact on marine life of higher atmospheric carbon dioxide, Ries wrote, "is more varied than previously thought."